How much of “Doctor Who” might really be possible?

Science shows why Doctor Who is so special.

As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS, and make changes to the past as and when he likes. But what is time travel – and how much of “Doctor Who” might really be possible?

A handy definition of time travel comes from philosopher David Lewis. Lewis says time travel involves a journey having different durations viewed from outside (in “external time”) or from inside (in “personal time”). Suppose you spend five minutes travelling aboard your machine, as measured by (e.g.) your watch and your memories. On arrival, you find 150 years have elapsed in the outside world. Congratulations, you have time-travelled. Five minutes of your personal time has covered 150 years of external time.

Odd as this sounds, Einstein’s theory of Special Relativity introduced such possibilities to physics in 1905. The theory says: the duration of a process varies with the relative velocity of the observer. The closer that relative velocity gets to the speed of light, the longer the travelling process takes.

Suppose you want to see the Earth a billion years hence, but worry you have only about 50 personal years left. Special Relativity specifies that if you travel very close to the speed of light relative to the Earth, your 50 personal years can cover one billion Earth years.

In backward time travel, personal and external time differ in direction, so journeys end in external time before, not after, they begin; you spend five personal minutes travelling 150 years into the external past. General Relativity suggests that the universe is essentially curved spacetime, which might allow such divergences of external and personal time.

Relativity treats space and time as aspects of a single entity: “spacetime”. One of the more remarkable features of General Relativity is that it allows time and space axes to be interchanged, so one observer’s space axis can be another observer’s time axis.

In 1949, Austrian mathematician Kurt Gödel used General Relativity to describe a universe where intrepid voyagers can go anywhere in (past or future) time without travelling faster than light. Gödel’s universe has no boundaries in space or time, and all the matter in it rotates. But our finite, non-rotating universe is not Gödel’s. Despair not though – simply spin an ultradense, very (maybe infinitely) long cylinder very fast. Spacetime should curve around the cylinder so the direction of the local future partially points into the external past. Such devices are called “Tipler Cylinders”, after physicist Frank Tipler.

Better yet, quantum theory suggests that “wormhole” connections between different spacetime points spontaneously form and break all the time. The chances are that natural wormholes are tiny - vastly smaller even than an electron, (and a billion trillion electrons can fit in a teaspoon). But you could perhaps find (or create) a wormhole big enough and durable enough to let you slip through into the past. Difficult, but theoretically possible.

No, you can’t kill your physics teacher

So perhaps you could travel into the past. But what about paradoxes? What is to stop you assassinating your grandfather or yourself as infants? One answer says: logical consistency.

Classical logic says you cannot consistently kill in infancy someone who achieves adulthood. But, Lewis says, time travel need not involve doing the logically impossible – provided travellers’ actions in the past are consistent with the history whence they come. So you could try killing your baby grandfather, but something would foil you – you would sneeze, or your gun would jam. Lewisian time travel is therefore (classically) consistent, but might look very strange, since seemingly possible actions (like shooting an unprotected infant) would prove impossible.

Another view says that backward time travel requires many worlds – that is, many different but equally real versions of physical reality. Physicist David Deutsch and philosopher Michael Lockwood argue that time travel must involve inter-world travel. If you travel backwards in time, you must arrive in a history different from your native one and so would be quite unfettered by your past once you get there. You could even kill this other history’s counterparts of your grandfather and yourself.

Both these concepts of backwards time travel may disappoint anyone wanting to change the “one and only” past. Conventional logic says time travellers would either help make history what it was (Lewis) or create a different history (Deutsch/Lockwood). However, quantum logic might let travellers change the actual (one-and-only) past.

Suppose we hold that quantum measurements determine (or change) quantities measured, even if those quantities lie in the past. Someone could travel back and “observe” history turning out differently from how it originally was, thereby retrospectively making actuality different from what it had been. What would happen to travellers who rebooted history is not clear, but this model seems closer to the time travel familiar from “Doctor Who” and other fictions. Beware, though, because quantum theory allows no predicting, and still less controlling, of the outcomes of changing the past. There would be no way to foresee the effect you would have on the present.

So classical logic, General Relativity and quantum theory all seem to permit time travel. Classical logic plus General Relativity suggest backward travellers face weird consistency constraints. Many-worlds travellers face no constraints, but get displaced into different histories. Quantum-logic travellers could change the (one and only) past without constraints, but they couldn’t predict or control what they would get.

So far, however, it seems only the Doctor knows how to change the past at will.

Alasdair Richmond received funding from the Arts and Humanities Research Council, to fund one semester of a two-semester leave period that covered the academic year 2008-2009.

The Conversation

This article was originally published at The Conversation. Read the original article.

The eleven incarnations of the Doctor. Image: BBC/Matt Burlem

Alasdair Richmond is a Senior Lecturer in Philosophy at the University of Edinburgh.

ED THOMPSON / LUZ / EYEVINE
Show Hide image

"We’ve got things in common": why one of the EDL's original members quit

An early supporter of the group, painter-decorator Darren Carroll has had death threats since he left. But why did he change his mind about the English Defence League?

Darren Carroll is a slight man with bright blue eyes and an urgent need for redemption. A painter-decorator in his fifties, he has lived in Luton his whole life. He was one of the original members of the English Defence League (EDL), the far-right street movement founded by Carroll’s nephew Tommy Robinson.

Recently, things haven’t been easy. Four months before our meeting at a café near Luton Airport Parkway Station, Carroll had a minor stroke that affected his speech and vision. It was the delayed fallout from an attack in a pub across the road, his local. A stranger, who seemed to know a lot about him, started a conversation. “He showed me his arm. It was tattooed. There was a little bit of white skin left on the whole sleeve,” says Carroll. “He said, ‘Look at that.’ I said, ‘What?’ He said, ‘White is right.’ I said, ‘Nah, mate, I know exactly where you’re coming from. There’s nothing wrong with being white but there’s nothing right with it.’”

The man pretended to leave the pub, then walked back in and hit Carroll hard on the back of the head with his forearm. Afterwards, Carroll suffered persistent headaches. It caused a blood clot that set off the stroke. When we met, he had mostly recovered but was still unable to work.

It was not the first attack. Carroll has also had his front door kicked in. He and his children have received death threats. “This is since speaking up,” he says. “Not leaving – that’s different.”

Carroll looks uncomfortable when we discuss the early days of the EDL. “It was an organic thing,” he says. “Lots of people were involved at the very beginning for different reasons. Personally, I was not happy with the way the town was being run on a political level. Looking back, I was disenfranchised from mainstream politics.”

Luton has the dubious distinction of being a centre of both far-right and Islamist extremism. The EDL began here in 2009, in response to a demonstration organised by Anjem Choudary’s now banned extremist group al-Muhajiroun, which in turn was a reaction against an army regiment marching in Luton.

A counterprotest led to arrests and the EDL was born, with sometimes violent neo-fascist street protests spreading across the country. Robinson insisted from the outset that the EDL was not racist, but only “against the rise of radical Islam”. Carroll says it was local difficulties, rather than national issues such as immigration, that unsettled and motivated him – and he didn’t articulate the core problem as racism against white people, not even to himself. The EDL has never had a formal membership, but the think tank Demos estimated that there were between 25,000 and 35,000 active members in 2011, a loose coalition of football hooligans and far-right activists. Today, the numbers are much reduced.

Carroll’s family was closely involved and it was a while before he realised that the EDL was an extremist, racist group. He describes being at a demo in Birmingham soon after the first protest. “I looked at the other lads there and I didn’t like them. They didn’t smell right for me, as far as integrity goes. I thought, ‘I don’t want this.’” Carroll’s parents are Irish and he considers himself the child of immigrants.

It took several months for him to extricate himself from the group and stop attending demonstrations. “It’s a relationship breaker, so you’ve got to accept that things are broken for ever.” On building sites, he was known as the EDL guy. Work dried up.

Amid attempts to coerce him back into the movement, and concerned about damaging his family relationships, Carroll stayed silent for another year and a half, only starting to speak up a few years after he left the EDL. This triggered a new wave of threats. He reeled off a list of incidents: slashed tyres, smashed windows. “Last week, I got one on Facebook [saying] that I’m a ginger Muslim and I’m gonna get shot. That was someone I know privately, which I don’t take as a threat. Their particular problem seems to be that I’m on record saying I’d have a cup of tea in a mosque and sit down and talk to people.”

Carroll did so after seeing a Facebook post by a local activist, Dawood Masood. Masood had shared a video of an imam in Leicester speaking about terrorist violence, with a message saying that any EDL members were welcome to get in touch. Carroll met him and others from the Muslim community and they discussed ways to make Luton better. He told them that he wasn’t interested in religion, but invited them to what he considers his church: Luton Town FC.

“I had the idea it’s about setting precedents, because you never know who or what that affects,” he says. “I just thought, if I’m seen going to the football with them, it’s going to break a big piece of ice.”

As the EDL evolved largely from a football subculture, this was a bold step. They went to the match. “He’s Luton born and bred and he certainly don’t need his hand held. But I made him as comfortable as possible. Luton scored and he’s jumping up and down, loving it. At that point, I thought: ‘This is really Luton harmony. He’s cheering for the same thing and I’m cheering for the same thing. We’re both happy together at this moment in time. We’ve got things in common.’”

They have been to many matches since, Masood bringing his kids, Carroll his grandkids. Carroll has had a few threatening calls but remains undeterred. “The working-class Muslim lads are working-class Muslim lads. They’ve got all the same problems and social issues as us white, working-class people. It’s not just me or us. It’s everyone.” 

Samira Shackle is a freelance journalist, who tweets @samirashackle. She was formerly a staff writer for the New Statesman.

This article first appeared in the 01 December 2016 issue of the New Statesman, Age of outrage